Combustion research performed in the Combustion Laboratory has obtained important results based on the use of spectroscopy techniques and CCD imagining techniques. These results, obtained at laboratory level, have characterized different flame types according to the emission of free radicals and shooting particles [see Farias, Combustion Control in a Fuel Oil Boiler Frame the Flame Spectrum (1997) the disclosure of which is incorporated herein by reference.]
Citing Zuo (1992), Tartari & Ngendakumana, [A Photodiode for Combustion Control, (2003) the disclosure of which is incorporated herein by reference.] present the application of photo diodes to detect radicals at particular wavelengths in order to define combustion control variables. This work, made in the laboratory of Thermodynamics of the University of Liége, Belgium in 2002, corresponds to one of the first investigations in the application of photo diodes to sense the photons of specific radicals. The used device corresponds to the combination photo diode-amplifying JIC 1EI17, which can be used externally with fixed or adjustable gain. The photo diode is sensitive in the 210-380 rank nm (UV) and has been used without interference filters. Due to the produced optical phenomena and the lack of rigor in the construction of the electronics responsible for device handling, the results were unsatisfactory.
On the other hand, it has been determined that CCD cameras have a linear spectral response and that this answer depends on the spectral sensitivity of the different sensors or filters for the colors of each camera individually. Additionally, they linearly depend on the incident power density and the wavelength. Finally, the spectral response is affected by noise, which can be diminished with a correct calibration. [Vora, Farell, Tietz and Brainard, (2000)].
Studies of the different color models and the advantages or disadvantages in their use of in different applications exist. The best known color system is the RGB because a computer program can easily decompose an image into the 3 channels of red, green, and blue. Furthermore, these channels are correlated to each other and certain procedures that require independence of the different color channels cannot be performed [Angulo, (2005)].
The HSB (Tonality, Saturation and Brightness) color model does allow this interdependence between the channels and it is very useful for procedures to recognize patterns in images. Additionally, the L*a*b* color model, which also allows us to obtain dependency between the channels, has also been studied [Ward, (2006)].
Several image processing strategies to identify certain indices characteristic of the flames have been developed, making control possible. Furthermore, certain correlations are determined between the luminous signal intensity captured with CCD cameras through their R, G and B filters, and some burner combustion or adjustment parameters. It is concluded that the excess of air in the combustion is correlated with the luminous signal intensity, explaining why these strategies can be used in combustion control. [Boysen, (2004)]
There are two commercial products, Spectra Tune and Flame Doctor, oriented to combustion optimization using optical signals.
SpectraTune is a combustion diagnosis and optimization system based on the analysis of spectral frequency, the visible phantom and the near TO GO, developed by Mark J. Khesin through the company Physical Sciences Inc. At present, the system is commercialized by GE Power Systems under the denomination “MK Optimization Combustion System”. Its design is limited to applications in power stations that use pulverized coal and its application depends on operator criteria and does not allow total automatization for burner control. Its cost is around US$ 25,000 for each Burner.
Flame Doctor is a system that provides real time analysis for each burner of a furnace. It analyzes the individual burner performance, identifying those with low yield and provides a diagnosis that orients the necessary adjustments. Flame Doctor uses signals from optical flame sensors, which with mathematical professing tools determine the degree of deviation with respect to the optimal degree. Its application has been developed for coal combustion and its price is in the order of US$ 150,000. The mentioned technologies are oriented to large-size facilities (on 350 MW).
Patent GB1396384 describes a sensor and a furnace control method that uses photoelectric sensors with two attachment lines to intercept the flame, producing two electrical exits or signals (x and y). The system consists of two tubes similar to a telescope, where the photodetector provides the optical image. Two types of sensors can be used: photovoltaic sensors if the fuel is coal or oil and infrared photodetectors if the fuel is natural gas. The signals x and y are processed electronically using a band pass filter of 200 Hz to 1000 hz, post-amplified, rectified and finally the signal is smoothed in a new filter. The time required is between 0 and 5 seconds, and action on the fuel source is soon taken when the flame is not detected.
The system correlates the information in three levels: the first one indicates flame absence, the second one indicates a normal flame, and the last level indicates an intermediate level of abnormal flame. The simple use of these optical radiation detectors (infrared, visible or ultraviolet) is believed to give a false indication about to the flame presence because they receive additional radiation from the furnace walls and a neighboring flame. In a large furnace, there are many burners and it is difficult to have a line of view of only one radiation, considering especially that the flame morphology depends on the fuel source as well as other factors. For these reasons, although devices sensitive to radiation amplitude can work satisfactorily with a single burner, the high level of background radiation disables the use of such devices in a furnace with multiple burners.
This circuit can not only distinguish the flame type, it can also be used to obtain the air-fuel relation, which depends on the flame position and is also influenced by the provided air flow. Therefore, this correlation can be used to obtain a warning device or a control system if the obtained correlation is outside certain range values.
This publication's detectors use the particular characteristics of the flames. One detector is a differentiated system in which two photoelectric cells are placed to observe the dark and light flame areas, respectively. Such a detector is applicable to coal flames where there is a dark area associated to the pulverized coal. Nevertheless, such a detector is very sensitive to changes in combustion conditions that modify the distance along the flame axis in which the fuel ignites. With a differentiated flame detector, the blockage of one of the vision tubes could even cause a differentiated signal when the flame is absent.
This patent publication uses photoelectric sensors to detect the flame presence and thus to act on the fuel source (three levels: without flame, normal flame and intermediate or abnormal flame) in order to provide flame monitoring when there is more than one burner, avoiding fuel injection when the flame has been extinguished in order to avoid possible explosions.
U.S. Pat. No. 4,435,149 describes a sensor and flame control method in a furnace, which manipulates the air/fuel variable in order to maintain maximum furnace efficiency. This sensor is based in the use of a radiometer with an infrared detector to observe the flame. The control system is based on the radiation signals emitted at three different wavelengths, using suitable filters.
The control parameter uses the ratio of at least two signals of infrared radiation to control the fuel/air mixture, maintaining furnace operation at its maximum efficiency. This publication uses a radiometer to detect the intensity emission in the flame's infrared range. The use of the radiometer in the visible spectral radiation range could increase the costs.
British patent GB1032697 publication describes a device for safety control of the flame in gas burners equipped with a pilot flame to ignite the main flame. This control device includes two flame detectors (one normally near to the flame pilot and another one near to the region of the main flame) and the circuit controls the main gas valve, whereas both detectors respond to the flame presence.
There are many flame detectors for this type of application, such as thermocouples, photoelectric cells, elements that respond to flame or gas conductivity and photodiodes in the ultraviolet or infrared range. Therefore, this device is used to maintain a safe flame operation.
U.S. Pat. No. 4,455,656, also discloses a combustion control circuit that includes the integrated circuits of a semiconductor used to control the furnace to warm up water, air or similar. This is a conventional combustion control circuit that uses the temperature signal, which means that the operation exit signal is adjusted using the voltage provided from the source to initialize the circuits.
U.S. Pat. No. 4,461,615, discloses a combustion control device based on the oxygen content, using an electrode in contact with the flame. Depending on the oxygen content and the source's voltage, the detection circuit produces a voltage that is compared with a reference voltage, and this information is used to control the fuel source's valve. The detection system is an intrusive method and the control is applied on the fuel flux. Thus, the control strategy does not consider combustion quality, which in addition to the power control (depending on the fuel flow) is given by air adjustment.
U.S. Pat. Nos. 4,553,924 and 4,509,912 disclose a system that uses an axis union to control the combustion in a furnace in order to maintain an optimal air/fuel mixture relation for all furnace power levels. The system consists of an adjustment of the mechanical connection in which a main arm is connected with an axis to control fuel valves and an auxiliary arm is connected to the air ventilator. This mechanical adjustment establishes a master-slave relation between the fuel and air valves.
U.S. Pat. No. 4,362,499 discloses a combustion control system for a furnace based on the monitoring of content of oxygen and carbon monoxide contents as well as the smoke's temperature in the combustion chamber. This system uses a conventional combustion control circuit based on gas concentration. The measurement parameters are used to calculate the on-line heat loss associated to combustion products. Based on the heat losses, the combustion air is controlled, minimizing heat losses and maximizing the furnace's thermal efficiency under different operation conditions. The disclosed system basically corresponds to a system of conventional combustion control based on the gas concentration and temperature, which corresponds to an invasive method.
Japanese Publication JP60036825 describes a combustion control system based on measuring temperature distributions in a flame without contact. The spectral analysis carried out by a light detector, and then the vibration spectrum of the OH radical is calculated to determine the flame's temperature distribution. This temperature is compared with an optimal distribution stored in the system, and this difference is then used to control the system. Only the radiation of the OH radical is used to determine the temperature distribution and the control action is based on this estimated temperature. The analyzed request is quite different to the proposed method, and thus does not affect this novelty.
U.S. Pat. No. 4,043,742 describes a combustion control system that uses radiation intensity at different, non-specified wavelengths. This ratio is then used by a master controller to regulate the air-fuel ratio. The detector is a complex system consisting of several mechanical and electro-optical components. The detector's signals are used to calculate a ratio between the intensities of two wavelengths that are correlated with the air-fuel ratio.
U.S. Pat. No. 5,971,747 describes a system using several detection techniques, such as CCD cameras, photo detectors, and laser based systems. A neural network analyzes the images and characterizes the combustion flame. Other forms of sensors monitor and generate data signals defining selected parameters of the combustion process. All the signals are analyzed using a fuzzy-logic based system, which generates control signals defining adjustments to optimize the combustion process. This system has many measuring sub-systems requiring sophisticated data processing techniques, resulting in a very complex and expensive system. As a result, this solution is not attractive for small-scale combustion systems.
U.S. Pat. No. 5,794,549 describes a system based on the flame temperature measurement using solid state CCTV cameras in furnaces that operate with pulverized coal. This system involves an image processor, a monitor adapted to exhibit the processed information, and a controller that regulates air-fuel ratio. Its strategy is based on reaching the ideal furnace functioning by controlling NOx emissions since a linear relation exists between the above mentioned emission and the combustion temperature. Coal presents higher NOx emissions that are more temperature dependent than gas and oil. Therefore, this strategy used would not be valid for gas and oil flames
U.S. Pat. No. 5,263,851 discloses a control system used in a radiant pipe gas burner installed in an oven. This system is based on the integrated, rectified signal of the voltage given by a germanium photodetector that feeds from the flame's luminous signal transmitted across an optical fiber. The sign of the integrated voltage is related to the fluctuation of the flame's intensity. The control action in this system is exercised in a discrete range to identify incomplete combustion, finished combustion and flame absence conditions. This control system establishes a wide operation region (finished combustion) and does not allow identification of the ideal one.